A number of light guiding devices are known. These devices are employed for a range of functions including illumination, backlighting, signage and display purposes. Typically, the devices are constructed from a moulded or cast transparent plastic component, where a light source, such as a fluorescent lamp or a plurality of light emitting diodes (LEDs), is integrated by means of mechanical attachment at the edge of the transparent plastic component.
Common to all of these devices is the fact that light from the light source is guided through a transparent guide, typically made of plastic, by total internal reflection. For backlighting applications, light is emitted in a substantially perpendicular direction to that of the direction of propagation of the light within the transparent guide. This may be achieved through the light being directed so as to interact with scattering structures located within, or on the surface of, the transparent guide.
The integration of fluorescent lamps or LEDs to the edge of the transparent light guide is not a straightforward process and thus significantly increases the complexity of the production process for these devices. Achieving a good coupling of the light source and the light guide is essential to the optical performance of the device. In addition, edge coupling of the light sources may render these components susceptible to mechanical damage during both the production process and the normal use of the device.
Many backlights fall into the categories of “edge-lit” or “direct-lit”. These categories differ in the placement of the light sources relative to the output of the backlight, where the output area defines the viewable area of the display device. In edge-lit backlights, one or more light sources are disposed along an outer border or edge of the backlight construction outside the zone corresponding to the output area. The light sources typically emit light into a light guide, which has length and width dimensions of the order of the output area and from which light is extracted to illuminate the output area. In direct-lit backlights, an array of light sources is disposed directly behind the output area, and a diffuser is placed in front of the light sources to provide a more uniform light output. Some direct-lit backlights also incorporate an edge-mounted light, and are thus illuminated with a combination of direct-lit and edge-lit illumination.
FIG. 1 illustrates a known edge-lit light guide arrangement. LEDs (1) are arranged at the edge of a transparent polymer core light guide layer (2). Light (3) from the LEDs propagates by total internal reflection through the light guide layer and is scattered through approximately 90° by scattering structures, such as point like defects, (4) and exits (3a) the light guide layer. In the figure shown, the device is viewed from above as indicated; the main light output surface is indicated at (5) and the point like defects (4) are located on the opposite lower surface. The refractive index contrast between the core and surrounding air provides the guiding effect. Light scattered out of this type of structure is emitted from the top surface over a full hemisphere of output angles.
The use of LEDs in backlight units is becoming increasingly popular. A standard LED package generally includes a hard plastic protecting material which supplies a high degree of mechanical stability to a lead frame structure. The lead frame possesses first and second terminals referred to as the die attach lead and the isolated lead by which electrical power is supplied to the LED package. The single LED may be connected to both leads by wire bonds. In operation, the LED package assembly has power applied to the lead frame at either of the first and second terminals depending on which part of the LED is the anode and which part is the cathode. The plastic protecting material allows for the manipulation and bending of the lead frame leads for solder configuration. Various polymers have been used by various manufacturers as the protecting material in connection with the packaging of LED products. However, methods for protecting the LED die are limited because of the relatively fragile nature of the wire-bonded lead frame arrangement. The hard plastic protecting material is normally applied using a resin-transfer process, a casting process which uses optical casting resins rather than injection-mouldable polymers. Resin transfer is a low pressure process that has a low risk of damaging wire bonds. Most protecting materials used for LED production have very high refractive indices resulting in a high proportion of the light generated by the LED die being reflected back in to the material at the material/air surface interface.
It is an object of the present invention to provide, inter alia, a light guiding device that addresses one or more of the aforesaid issues.